Typically resistors in integrated circuits are formed in the polysilicon gate electrodes, particularly in various analog applications in 45 nm products. However, as the dimensions of transistor devices continue to shrink, various issues arise imposing increasing demands for methodology enabling the fabrication of semiconductor devices having high reliability and high circuit speed. Smaller transistors require reduced feature sizes. As the gate width for transistors decreases, the reduction in the polysilicon thickness, i.e., below 800 Å, negatively affects the resistance of the resistors.
An attractive alternative to poly resistors is RX based resistors (i.e., resistors formed in the active silicon regions of the silicon substrate), since the active silicon regions are not impacted by the scaling down of the transistor and resulting reduction in polysilicon thickness. In 32 nm technologies and beyond, transistors are typically formed with metal gates and high-K dielectrics, and the polysilicon is replaced with amorphous silicon (a-Si). Resistors formed from the a-Si are impaired because a-Si has an inferior temperature coefficient of electrical resistance (TCR) due to larger grain size. RX resistors have proven to be an attractive alternative, as they have exhibited an improved TCR, with a pre-amorphization implant.
The resistance of resistors is controlled by implanting silicon with boron (B). To implant poly resistors with boron (B) in 45 nm technology, the energy used is typically 8 keV, and the dosage ranges from 2.6E15 to 6.5E15/cm2. To use the same energy for targeting RX resistors, a lower B dose as compared with poly/a-Si resistors is required, for example 9E14/cm2, to attain the same sheet resistance as the poly resistors. However, the use of a low dose has been found to be problematic.
First, a low B dose decreases the local matching between close proximity resistors. A lower dose means fewer B atoms, which translates into a higher random dopant fluctation. Since local matching is driven by random dopant fluctuation, the local mismatch coefficient has a linear dependence on B dose. For the target B dose of 9E14, the local mismatch is expected to be about 2.1%-μm, which is about twice that for poly resistors. A solution for improving local matching is to increase the size of the resistor, thereby reducing random dopant fluctuation. However, increasing resistor size causes an area penalty to the design.
Another problem attendant upon employing a low B dosage is a higher end resistance, because the silicide to silicon contact becomes a Schottky contact instead of ohmic. For a B dose of about 1.1E15, the end resistance is about 60 ohms-μm, which is significantly higher than the upper specification limit of 45 ohms-μm. High Rend also impacts the VCR of the resistors. A lower dose, such as the target 9E14, raises the end resistance even further above the specification limit. A proposed solution for improving the end resistance is to increase the implants at the resistor ends. However, that would require an undesirable mask change for existing designs to allow additional implants only in the resistor ends, thereby decreasing manufacturing throughput and increasing cost. It would also adversely affect the sheet resistance of small length resistors.
A need therefore exists for efficient methodology enabling the fabrication of semiconductor devices having RX resistors with increased local matching between close proximity resistors, improved end resistance, and reduced random dopant mismatch.